In Settings - General - Currencies it is defined if the currency is always EUR (Stationary). With selection Stationary, only one currency is used on the invoice. If there are several currencies in use, To choose is selected here. Then EUR is the default currency but also other currencies can be used. You can also define a default currency for the supplier in the supplier register (Registers - Suppliers). The average exchange rate of European Central Bank is used on the invoices.
The currency list shows the most common currencies by default. If you want to add more currencies, you can select them from the dropdown list in the end. Please note that there can be up to 30 currencies selected on the list. The currencies that are not needed can be removed by clicking the blue cross in the currency box.
In the end save the information with Save button. NOTE! All the tags in General Settings can be filled before saving.